The invention discloses a safety belt hanging method for an underground diaphragm wall. A safety belt hanging bracket arranged on a foundation pit enclosure wall under construction is adopted to hanga safety belt worn by construction personnel; the foundation pit enclosure wall under construction comprises an underground diaphragm wall and a water-retaining wall arranged on the underground diaphragm wall; and the safety belt hanging bracket comprises a safety rope connected with the safety belt, a movable bracket, a safety rope winding mechanism, a safety rope anti-reverse mechanism and a safety rope guiding mechanism. When the safety belt is hung, the method comprises the steps that first, a construction platform is erected; second, the safety belt hanging bracket is moved into place; and third, the safety belt is hung. The safety belt hanging method for the underground diaphragm wall is simple, the design is reasonable, and the use effect is good; and the safety belt hanging bracketwhich can be moved simply and conveniently and can be firmly fixed is adopted to provide a safety belt hanging point which is flexible in position, simple and convenient for the hanging of the safetybelt hanging point, and safe and reliable, so that the safety belt hanging requirement during large-area manual chiseling operation of the underground diaphragm wall can be effectively met, and it isensured that the construction process is safe and reliable.